Automatic slice prescription techniques are nowadays commonly used during Magnetic Resonance Imaging (MM) scanning procedures. An anatomical localizer scan is acquired as the first scan of the session, and image processing techniques are used to determine a set of patient examination (frame-of-reference) specific anatomical references, for example, by identifying landmarks in the images. These anatomical references, which may contain geometric transformation matrices and body part boundary information, are stored in memory.
Based on the anatomical reference, the system automatically transforms subsequent atlas-related scan prescriptions into individual patient anatomy related slices and field-of-view prescriptions. This leads to more consistent field-of-view (FOV) placement and image orientations across patients and also ensures consistent FOV placements and image orientations for follow-up scans of the same patient. This technique has been implemented in similar fashion by various vendors (e.g., AutoAlign by Siemens, SmartExam by Philips, ReadyBrain by GE and NeuroLine/SpineLine by Toshiba) for imaging different anatomical region (e.g., brain, spine, knee etc.). FIG. 1 provides a schematic description of how, in practice, an anatomical localizer may be used for automatic FOV adjustment of subsequent scans based on anatomic landmark detection.
If the patient moves at any stage after the anatomical localizer is acquired the (frame-of-reference related) anatomical references are no longer valid and FOV placement for all subsequent scans after the motion occurs is inconsistent. FIG. 2 illustrates this inconsistency. As shown in this example, the patient motion after scan 1 makes FOV placement for scans 2 to N inconsistent since the anatomic landmarks calculated from the anatomical localizer are no longer valid. In some instances, automatic correction of the positioning throughout the scan session can also be used to ensure that when the technician places an ROI (FOV) on the initial localizer (3-plane or volume), it remains valid even if the patient moved since the localizer was acquired. The localizer doesn't have to be reacquired in such a case, but the positioning should be corrected for the intervening motion since that time so that the FOV is still as intended by the technician based on anatomy.
In conventional systems, patient motion is addressed by discontinuing the current frame-of-reference and to clear the memory of all anatomical references. Then, the localizer with the auto-detection of anatomical references has to be repeated and all slice prescriptions in the remaining scans have to be updated before finishing the examination. Any geometric relation between the two frame-of-references is lost. This approach has major disadvantages. First, the decision to re-run the anatomical localizer scan is taken by the operator whenever patient motion is observed e.g. with a monitoring camera. This is a not a foolproof method since small patient motions are difficult to spot, and depending on the setup there may not be a line of sight vision to the anatomy being scanned. The second disadvantage is that the anatomical localizer scan typically takes quite a long time so repeating this between scans is not efficient in clinical routine. Finally, with the conventional technique, it is difficult to overlay images acquired before and after this update as their geometrical relation is wrong. In some applications, this may, in turn, require a reacquisition of multiple series or of the whole examination.